Pressure swing adsorption is a well-known method for the separation of bulk gas mixtures and for the purification of gas streams containing low concentrations of undesirable components. The method has been developed and adapted for a wide range of operating conditions, product purity, and product recovery. Many pressure swing adsorption (PSA) systems utilize two or more adsorber beds operated in a cyclic sequence in order to maintain a constant product flow rate while selected beds undergo various steps including adsorption, depressurization, desorption, purge, pressure equalization, pressurization, and other related steps. Multiple adsorber beds using numerous process steps are required to achieve high purity and/or recovery of valuable gaseous products such as hydrogen, carbon oxides, synthesis gas, light hydrocarbons, and the like. The high cost of generating the feed gas mixtures containing these valuable components usually justifies the complexity and capital expense of multiple-bed PSA systems.
Other gaseous products amenable to recovery by PSA do not require the high purity and/or recovery of the above-named products. In the recovery of oxygen and nitrogen from air by PSA, for example, lower purity products are acceptable for certain end uses and simpler PSA systems can be used for such products. These simpler PSA systems have significantly lower capital and operating costs than the multiple-bed systems earlier described. The simplest of these PSA systems for air separation utilize a single adsorber bed in conjunction with one or more gas storage vessels to allow constant product flow and provide gas for adsorber purge and pressurization during the regeneration portion of the PSA cycle. A number of single-bed PSA processes have been developed and are known in the art.
U.S. Pat. No. 4,561,865 discloses a single-bed PSA system comprising an adsorber and a surge tank operated with a feed compressor in a three-step cycle. First, compressed feed air is introduced into the adsorber, which increases the pressure in the adsorber, and simultaneously adsorber effluent is withdrawn into the surge tank. A portion of the gas is withdrawn from the surge tank as an oxygen-enriched product. The adsorber feed is then discontinued and the adsorber is vented countercurrently (i.e. through the adsorber feed end) to the atmosphere. During this venting step, purge gas from the surge tank is introduced into the product end of the adsorber. Upon completion of the vent/purge step, the adsorber and the surge tank are pressure equalized through the adsorber product end (i.e. countercurrently). The steps are repeated in a cyclic manner. U.S. Pat. No. 4,511,377 describes a modular apparatus using this PSA process.
A single-bed PSA system is described in U.S. Pat. No. 4,892,566 which utilizes an adsorber in conjunction with a surge tank, feed compressor, and switch valves to carry out a series of steps. First, compressed feed air is introduced into the adsorber, which increases the pressure in the adsorber while at the same time adsorber effluent is withdrawn into the surge tank. A portion of the gas is withdrawn from the surge tank as an oxygen-enriched product. The adsorber feed is discontinued and the adsorber outlet closed, and the adsorber is vented countercurrently (i.e. through the adsorber feed end) to the atmosphere. Gas from the surge tank is introduced into the adsorber countercurrently (i.e. through the adsorber product end) and the pressures in the adsorber and surge tank are equalized. The adsorber is then pressurized with feed air through the feed end and pressure equalized with the surge tank. The adsorber is further pressurized to a pressure above that of the surge tank, and finally the adsorber and surge tank are pressure equalized. The steps are then repeated in cyclic fashion.
U.S. Pat. No. 5,032,150 discloses a single-bed PSA process which utilizes multiple gas storage tanks in a PSA cycle to separate air. Compressed air is fed from an air feed tank into an adsorber presaturated with oxygen-rich gas from a previous cycle and the adsorber effluent is directed into a product collector tank, from which a portion of the gas is withdrawn as an oxygen-rich product. The adsorber outlet is then closed and the adsorber is pressure equalized with the air feed tank. Next, the adsorber is rinsed with nitrogen-rich gas from a nitrogen product tank, and the displaced gas is stored in the air feed tank. The nitrogen-saturated adsorber then is depressurized countercurrently (i.e. through the adsorber feed end) into the nitrogen product tank. Nitrogen may be withdrawn as a product if required. Finally the adsorber is purged countercurrently with oxygen-rich gas from the product collector tank to displace the nitrogen therein and then is pressurized countercurrently with the oxygen-rich gas to the adsorption pressure. The steps are repeated in a cyclic manner.
A single-vessel rapid PSA system is described in U.S. Pat. No. 5,071,449 in which the vessel contains dual adsorption layers and operates in alternating fashion with a continuous feed gas and two continuous product streams. A product surge tank is not used. Another rapid PSA system utilizing a single adsorbent bed operating in a cycle of 30 seconds or less is described in U.S. Pat. No. 4,194,892. The adsorber effluent optionally flows through a product surge tank to dampen flow fluctuations during adsorber cycling.
A single-bed PSA system with a product surge tank and an equalization tank is disclosed in U.S. Pat. No. 5,370,728. In the operation of this system, compressed air feed is introduced into the adsorbent bed, pressurizing the bed from an intermediate pressure up to a maximum adsorption pressure, and the effluent product is withdrawn from the bed into the product surge tank. The adsorbent bed then is isolated and depressurized cocurrently (i.e. through the product end) into an equalization tank at the intermediate pressure. Next, the bed is further depressurized countercurrently (i.e. through the feed end) to a lower desorption pressure, and the bed is purged countercurrently with gas from the product surge tank. The bed is then pressurized countercurrently to the intermediate pressure with gas from the equalization tank. Finally the bed is pressurized with feed air and the steps are repeated in a cyclic manner.
Single-bed PSA systems for generating an oxygen-enriched gas from air can be utilized efficiently as described in the art summarized above. Improvements to the simplest of these single-bed systems are advantageous and will encourage the wider use of such systems for the supply of moderate-purity oxygen or nitrogen. The invention described below and defined in the claims which follow offers an improvement to a single-bed PSA system for providing such products.